Rollup merge of #41266 - projektir:weak_docs_rc, r=alexcrichton

Updating docs for std::rc::Rc

The same changes as PR [#41240 ](https://github.com/rust-lang/rust/pull/41240), but for [`std::rc::Weak`](https://doc.rust-lang.org/std/rc/struct.Weak.html). At least, as far as I am aware, the Weak pointer is the same for both, and they're basically the same, just one is thread-safe and the other is not.

r? @alexcrichton